Metformin (INN; trade names Glucophage, Riomet, Fortamet, Glumetza, Diabex, Diaformin, and others) is an oral anti-diabetic drug from the biguanide class. It is the first-line drug of choice for the treatment of type 2 diabetes, particularly in overweight and obese people and those with normal kidney function. Metformin is the most popular anti-diabetic drug in the United States and one of the most prescribed drugs in the country overall, with nearly 35 million prescriptions filled in 2006 for generic metformin alone. As of 2007, it is one of only two oral anti-diabetics in the World Health Organization Model List of Essential Medicines (the other being glibenclamide).

The biguanide class of anti-diabetic drugs, which also includes the withdrawn agents phenformin and buformin, originates from the French lilac (Galega officinalis), a plant known for several centuries to reduce the symptoms of diabetes mellitus.

Metformin was first described in the scientific literature in 1957. It was first marketed in France in 1979, but did not receive approval by the U.S. Food and Drug Administration (FDA) for Type 2 diabetes until 1994. Bristol-Myers Squibb's Glucophage was the first branded formulation of metformin to be marketed in the United States, beginning on March 3, 1995. Generic formulations are now available.